The present invention relates to a method for preparing a catalyst for oxidation of methanol to formaldehyde and to its use in processes for preparing formaldehyde.
Catalysts for the oxidation of methanol to formaldehyde (commonly termed “iron molybdate”, since Fe2(MoO4)3 is one of the main active components) comprise a Fe2(MoO4)3/MoO3 mixture in which the Mo/Fe ratio is comprised between 1.5 and 5, and are generally prepared according to a method which comprises the precipitation of the above catalytic mixture from an aqueous solution of a soluble ferric salt, such as FeCl3, Fe(NO3)3 and the like, mixed with a solution of an alkaline molybdate or of ammonium, subsequent dilution of the suspension, filtration and washing of the precipitate, its conversion into a slurry by agitation, drying of the slurry and subsequent shaping of the dried powder or of a paste thereof so as to obtain granules having a specific geometric shape, and calcination of the granules at temperatures generally from 450 to 550° C.
Precipitation is performed from solutions with relatively low pH values (comprised from 1 to 2 in the case of precipitation from ferric chloride solutions), at which there is a significant solubilization of iron molybdate.
The discharge of the mother liquors and wash water containing molybdate ions as well as ammonium, ferric, nitrate and chloride ions becomes problematic, because these ions are pollutants and statutory provisions currently in force in many countries prohibit their discharge or allow it at concentrations far lower than those of the mother liquors and/or wash water.